The present invention relates to speed ratio control of a vehicle equipped with a slip control system and a continuously variable transmission (CVT).
When slip control systems, such as an anti-lock brake system (ABS) or a traction control system (TCS), are operating, a braking force or drive force fluctuates and a sensor detected vehicle speed (=drive wheel speed) fluctuates. Therefore, if speed ratio control is performed based on the sensor detected vehicle speed, a speed ratio may vary suddenly. If the speed ratio varies suddenly during operation of a slip control system, the effectiveness of slip suppression by the slip control system will decline.
In JP-A-H2-234851 published by the Japanese Patent Office in 1990, a technique is disclosed of preventing a sudden change of speed ratio so as to prevent the decline of slip suppression by performing speed ratio control based not on the sensor detected vehicle speed, but on an estimated vehicle speed (=driven wheel speed).
However, when the slip control system stops, the aforesaid speed ratio controller immediately changes over the vehicle speed used for speed change control from the estimated vehicle speed to the sensor detected vehicle speed. When the difference between the estimated vehicle speed and sensor detected vehicle speed is large, the vehicle speed used for speed ratio control varies sharply, and as a result the speed ratio varies sharply. This sharp variation of the speed ratio causes the vehicle to suffer a shock or causes it to slip again.
It is therefore an object of this invention to prevent a sharp variation of speed ratio when a slip control system has stopped, and to prevent the occurrence of shocks or the recurrence of slipping.
In order to achieve above object, the present invention provides a speed ratio control device used for a vehicle comprising a continuously variable transmission and a slip control system which controls a slip of a driving wheel by controlling either of braking force and driving force. The control device comprises a sensor which detects a vehicle speed, an actuator which changes a speed ratio of the transmission, and a microprocessor programmed to estimate a vehicle speed based on a running state, compute a target speed ratio of the transmission based on the sensor detected vehicle speed when the slip control system is not operating, compute the target speed ratio of the transmission based on the estimated vehicle speed when the slip control system is operating, and control the actuator so that the speed ratio of the transmission approaches the target speed ratio. The microprocessor is further programmed to determine whether, when the slip control system has stopped, a speed ratio variation amount due to a change-over of the vehicle speed used for speed change control from the estimated vehicle speed to the sensor detected vehicle speed, is less than a permissible amount, and when it is determined that the speed ratio variation amount is less than the permissible amount, to change over the vehicle speed used for speed change control from the estimated vehicle speed to the sensor detected vehicle speed.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.